The present invention is related to torque control of a vehicle, such as a locomotive or a transit vehicle, propelled by traction motors, and, more particularly, to a torque controller system for providing improved torque distribution in that vehicle.
Locomotives and transit vehicles as well as other large traction vehicles are commonly powered by electric traction motors which are coupled to drive one or more axles of the vehicle. Locomotives and transit vehicles generally have at least four axle wheel sets per vehicle with each axle-wheel set being connected via suitable gearing to the shaft of an electric motor commonly referred as a traction motor. In a motoring mode of operation, the traction motors are supplied with electric current from a controllable source of electric power, such as an inverter. The traction motors apply torque to the axles which, in turn, apply torque to the wheels of the vehicle. The wheels exert tangential force or tractive effort on the surface on which the vehicle is traveling (e.g., the substantially parallel steel rails of a railroad track). Alternatively, in an electrical braking mode of operation, the motors operate as axle-driven electrical generators; that is, torque is applied to the motor shafts by their respectively coupled axle-wheel sets which then exert braking effort on the surface, thereby retarding or slowing the motion of the vehicle.
For efficient operation, either in the motoring or in the braking mode of operation, the vehicle is required to provide a substantial level of adhesion between its wheels and the surface on which the vehicle is traveling. In view of that requirement, the vehicle is generally required to achieve the maximum reachable adhesion on every axle-wheel set while, due to cost considerations, the respective power ratings of the controllable power source, the traction motor, wiring, and other equipment coupled to drive each axle-wheel should be as low as feasible for a given application. Due to various factors, such as wear and tear, or improper maintenance, the size of the diameter of the vehicle wheels may change relative to one another. Such wheel diameter differences can produce unequal vertical force or weight on each axle. The unequal vertical force could also be due to dimensional variations on the platform or the trucks where respective ones of the axle-wheel sets are mounted. If, for example, the vertical force on a given axle-wheel increases due to one or more of the above-listed factors, then the available tractive effort on that axle-wheel would increase and this situation would require a higher rated power equipment to make use of the increased tractive effort. It will be appreciated that when the vertical force increases on a given axle-wheel, there is a corresponding vertical force reduction to the other axle-wheels since the total vertical force in the vehicle remains constant. The reduced vertical force in turn produces a reduction in available tractive effort on the other axle-wheels and thus the rating of the power equipment coupled to drive these other axle-wheels would be less relative to the power equipment coupled to the axle-wheel with increased vertical force. Thus, it is desirable to operate each axle-wheel set such that each requires substantially the same power rating relative to one another, and it is desirable that each axle-wheel produce substantially the same level of tractive effort under worst case operating conditions, that is, when maximum tractive effort is truly required.
Presently available torque controllers are generally configured so that the torque supplied to each axle-wheel set under normal steady state operating conditions is substantially the same relative to one another, regardless of the vertical force any given axle-wheel set actually receives. This substantially even torque distribution would make tractive effort on a smaller diameter wheel greater than on a larger diameter wheel and would cause the smaller wheel to wear out faster. Thus, it would be advantageous to have a torque controller system which would allow for distributing the tractive effort such that it would be greater on axles with larger diameter wheels than on axles with smaller diameter wheels, at least under normal operation, that is, not during operational conditions which require worst-case tractive efforts. Any shift or distribution of tractive effort to the larger axle-wheel would make a corresponding reduction in tractive effort to the smaller axle-wheels; consequently, there would be a reduction of the wear rate of those axle-wheels compared to the wear rate of the larger wheel. Tractive effort distribution which takes into account the actual tractive effort requirements of each axle-wheel set in the vehicle would advantageously result in eventually all of the wheels having a substantially similar diameter with respect to one another since during most operations the locomotive will be running at high speeds.